The present invention relates to an electric double-layer capacitor and, more particularly, to an electric double-layer capacitor which is characterized by a decreased internal resistance and an increased power density by reducing the contact resistance between each polarizing electrode and a corresponding aluminum collector electrode, the polarizing electrode consisting mainly of activated carbon.
The prior art electric double-layer capacitor can store a considerably smaller amount of energy than the lead-acid battery that is a typical secondary battery. However, the electric double-layer capacitor exhibits an electrostatic capacity density larger than that of an aluminum electrolytic capacitor of similar withstand voltage by several orders of magnitude. Furthermore, the electric double-layer capacitor stores electric energy as electron itself and so the capacitor can, in essence, store and release electric energy efficiently and quickly.
In view of these advantages, techniques for connecting such electric double-layer capacitors in parallel with a battery in applications where quick charging and discharging are required such as in electric vehicles have been investigated. Furthermore, electric double-layer capacitors having lower internal resistances and used in various power applications have been investigated.
FIG. 8 depicts the structure of a conventional electric double-layer capacitor, illustrating the internal resistance. The inside of the capacitor is partitioned into parts by a separator 5. Electrodes 3 of activated carbon are placed in an organic electrolyte 4 in these parts. Each electrode 3 is connected with an electrical lead 6 via a contact portion 2 and an aluminum collector electrode 1. Where the energy density of the electric double-layer capacitor is high and the internal resistance is relatively high (on the order of 102xcexa9 per electrostatic capacity F), the electrodes of activated carbon have a relatively large thickness of several millimeters. The impregnated electrolyte contributes mostly to the internal resistance. The electrodes of activated carbon contribute secondly. The effects of the aluminum collector electrodes and the contact resistance are very small.
However, in an electric double-layer capacitor having one order of magnitude less internal resistance, the thickness of the electrodes of activated carbon is roughly halved. Concomitantly, the contributions of the electrolyte and the electrodes of activated carbon are decreased accordingly.
In a type having one order of magnitude further less internal resistance, the electrodes of activated carbon are thinner. Therefore, the contributions of the electrolyte and the electrodes of activated carbon are still smaller. Consequently, the electrical leads and the aluminum collector electrode (especially, the contact resistance shown in FIG. 5) affect the characteristics of the whole capacitor materially.
Aluminum is a metal having a low electrical resistance. Furthermore, aluminum has the advantage that it deteriorates the withstand voltage of the assembled capacitor only a little. Collector electrodes of aluminum have unparalleled excellence. However, great problems take place where collector electrodes of aluminum are used. An oxide film is immediately formed on the surface. Even if this film is very thin, its withstand voltage is so high that a low voltage handled in electric double-layer capacitors cannot break the film. This film is inserted between the aluminum collector electrode and an electrode of activated carbon, producing contact resistance.
This oxide film of aluminum gives better withstand voltage to aluminum than nickel and titanium that are much more inactive. Therefore, the insulation provided by the presence of the oxide film and increase of the contact resistance are two conflicting factors.
Consequently, where various kinds of plating are formed on the aluminum film, the surface is processed to impart electrical conductivity to the surface, or adhesive bonding is done, if the contact resistance is successfully reduced, then the withstand voltage deteriorates. Conversely, if attention is paid to the withstand voltage, the contact resistance does not decrease.
The present invention is intended to solve the foregoing problem.
It is an object of the present invention to provide an electric double-layer capacitor which uses aluminum collector electrodes and has a reduced internal resistance, thus achieving higher power density.
The present invention lies in an electric double-layer capacitor comprising positive and negative polarizing electrodes separated by a separator and aluminum collector electrodes for extracting electric charge from the polarizing electrodes that consist mainly of activated carbon and produce electric double layers. The aluminum collector electrodes are connected with external terminals, and each has an oxide film on its surface. Hard granular carbon penetrates through the oxide film into the aluminum electrode. The aluminum electrodes are in contact with their respective polarizing electrodes via the granular carbon.
Other objects and features of the invention will appear in the course of the description thereof, which follows.
A complete understanding of the invention will be obtained from the following description when taken in connection with the accompanying drawing figures wherein like reference characters identify like parts throughout.